The Spatial Distribution of Chlorophyll in Leaves

Single-cell transcriptome landscape elucidates the cellular and developmental responses to tomato chlorosis virus infection in tomato leaf

Plant viral diseases compromise the growth and yield of the crop globally, and they tend to be more serious under extreme temperatures and drought climate changes. Currently, regulatory dynamics during plant development and in response to virus infection at the plant cell level remain largely unknown. In this study, single-cell RNA sequencing on 23 226 individual cells from healthy and tomato chlorosis virus-infected leaves was established. The specific expression and epigenetic landscape of each cell type during the viral infection stage were depicted. Notably, the mesophyll cells showed a rapid function transition in virus-infected leaves, which is consistent with the pathological changes such as thinner leaves and decreased chloroplast lamella in virus-infected samples. Interestingly, the F-box protein SKIP2 was identified to play a pivotal role in chlorophyll maintenance during virus infection in tomato plants. Knockout of the SlSKIP2 showed a greener leaf state before and after virus infection. Moreover, we further demonstrated that SlSKIP2 was located in the cytomembrane and nucleus and directly regulated by ERF4. In conclusion, with detailed insights into the plant responses to viral infections at the cellular level, our study provides a genetic framework and gene reference in plant-virus interaction and breeding in the future research.

Rice bundle sheath cell shape is regulated by the timing of light exposure during leaf development

Plant leaves contain multiple cell types which achieve distinct characteristics whilst still coordinating development within the leaf. The bundle sheath possesses larger individual cells and lower chloroplast content than the adjacent mesophyll, but how this morphology is achieved remains unknown. To identify regulatory mechanisms determining bundle sheath cell morphology we tested the effects of perturbing environmental (light) and endogenous signals (hormones) during leaf development of Oryza sativa (rice). Total chloroplast area in bundle sheath cells was found to increase with cell size as in the mesophyll but did not maintain a 'set-point' relationship, with the longest bundle sheath cells demonstrating the lowest chloroplast content. Application of exogenous cytokinin and gibberellin significantly altered the relationship between cell size and chloroplast biosynthesis in the bundle sheath, increasing chloroplast content of the longest cells. Delayed exposure to light reduced the mean length of bundle sheath cells but increased corresponding leaf length, whereas premature light reduced final leaf length but did not affect bundle sheath cells. This suggests that the plant hormones cytokinin and gibberellin are regulators of the bundle sheath cell-chloroplast relationship and that final bundle sheath length may potentially be affected by light-mediated control of exit from the cell cycle.

GhCTSF1, a short PPR protein with a conserved role in chloroplast development and photosynthesis, participates in intron splicing of rpoC1 and ycf3-2 transcripts in cotton

Cotton is one of the most important textile fibers worldwide. As crucial agronomic traits, leaves play an essential role in the growth, disease resistance, fiber quality, and yield of cotton plants. Pentatricopeptide repeat (PPR) proteins are a large family of nuclear-encoded proteins involved in organellar or nuclear RNA metabolism. Using a virus-induced gene silencing assay, we found that cotton plants displayed variegated yellow leaf phenotypes with decreased chlorophyll content when expression of the PPR gene GhCTSF1 was silenced. GhCTSF1 encodes a chloroplast-localized protein that contains only two PPR motifs. Disruption of GhCTSF1 substantially reduces the splicing efficiency of rpoC1 intron 1 and ycf3 intron 2. Loss of function of the GhCTSF1 ortholog EMB1417 causes splicing defects in rpoC1 and ycf3-2, leading to impaired chloroplast structure and decreased photosynthetic rates in Arabidopsis. We also found that GhCTSF1 interacts with two splicing factors, GhCRS2 and GhWTF1. Defects in GhCRS2 and GhWTF1 severely affect intron splicing of rpoC1 and ycf3-2 in cotton, leading to defects in chloroplast development and a reduction in photosynthesis. Our results suggest that GhCTSF1 is specifically required for splicing rpoC1 and ycf3-2 in cooperation with GhCRS2 and GhWTF1.

A fluorescence spectroscopic study of light transmission and adaxial-abaxial distribution of emitting compounds in leaves of Christmas star (Euphorbia pulcherrima)

In situ fluorescence measurements have been used to investigate relative amounts of blue-green pigments and their distributions in plant leaves from Euphorbia pulcherrima. Advantage was taken from the fact that this species has white leaves on the top, with low pigment concentrations, and green leaves on the stem with ordinary pigment concentrations. Excitation- and emission spectra below 410 nm from white leaves, where pigment absorption is low, are not distorted by self-absorption. Absorption- and reflection spectra from white and green leaves were measured using a spectrophotometer equipped with an integrating sphere. The absorption spectra were used to correct recorded fluorescence spectra for self-absorption. Self-absorption corrected photosystem fluorescence from green leaves, modeling light transmission in leaf tissue exponentially, matches to the excitation/emission spectra from white leaves, apart from small differences due to the pigment concentrations and selective scattering. The introduced exponentially decaying transmission relation also predicts that the ratio of excitation spectra from a white and green leaf is in proportion to the absorption spectrum of the green leaf, which was validated for Photosystem II particle fluorescence. This relation was also used to find a scaled absorption spectrum responsible for blue-green emission, which was assumed to originate from lignin. Excitation/emission spectra of the blue-green fluorescence were decomposed into five components and their relative amounts from adaxial and abaxial sides of the leaves have been quantified. Fluorescence lifetime measurements of the leaves, upon 403 nm excitation, revealed three decay times corresponding to the lignin fluorophores emitting in blue and green spectral region, and indicated that emissions at 500 and 550 nm may originate from the same fluorophore residing in the two physically different environments.

Novel 1,3,4-Thiadiazole Derivatives: Synthesis, Antiviral Bioassay and Regulation the Photosynthetic Pathway of Tobacco against TMV Infection

Tobacco mosaic virus (TMV) is a systemic virus that poses a serious threat to crops worldwide. In the present study, a series of novel 1-phenyl-4-(1,3,4-thiadiazole-5-thioether)-1H-pyrazole-5-amine derivatives was designed and synthesized. In vivo antiviral bioassay results indicated that some of these compounds exhibited excellent protective activity against TMV. Among the compounds, E₂ (EC₅₀ = 203.5 μg/mL) was superior to the commercial agent ningnanmycin (EC₅₀ = 261.4 μg/mL). Observation of tobacco leaves infected with TMV-GFP revealed that E₂ could effectively inhibit the spread of TMV in the host. Further plant tissue morphological observation indicated that E₂ could induce the tight arrangement and alignment of the spongy mesophyll and palisade cells while causing stomatal closure to form a defensive barrier to prevent viral infection in the leaves. In addition, the chlorophyll content of tobacco leaves was significantly increased after treatment with E₂, and the net photosynthesis (Pn) value was also increased, which demonstrated that the active compound could improve the photosynthetic efficiency of TMV-infected tobacco leaves by maintaining stable chlorophyll content in the leaves, thereby protecting host plants from viral infection. The results of MDA and H₂O₂ content determination revealed that E₂ could effectively reduce the content of peroxides in the infected plants, reducing the damage to the plants caused by oxidation. This work provides an important support for the research and development of antiviral agents in crop protection.

Light scattering in stacked mesophyll cells results in similarity characteristic of solar spectral reflectance and transmittance of natural leaves

Solar spectral reflectance and transmittance of natural leaves exhibit dramatic similarity. To elucidate the formation mechanism and physiological significance, a radiative transfer model was constructed, and the effects of stacked mesophyll cells, chlorophyll content and leaf thickness on the visible light absorptance of the natural leaves were analyzed. Results indicated that light scattering caused by the stacked mesophyll cells is responsible for the similarity. The optical path of visible light in the natural leaves is increased with the scattering process, resulting in that the visible light transmittance is significantly reduced meanwhile the visible light reflectance is at a low level, thus the visible light absorptance tends to a maximum and the absorption of photosynthetically active radiation (PAR) by the natural leaves is significantly enhanced. Interestingly, as two key leaf functional traits affecting the absorption process of PAR, chlorophyll content and leaf thickness of the natural leaves in a certain environment show a convergent behavior, resulting in the high visible light absorptance of the natural leaves, which demonstrates the PAR utilizing strategies of the natural leaves. This work provides a new perspective for revealing the evolutionary processes and ecological strategies of natural leaves, and can be adopted to guide the improvement directions of crop photosynthesis.

Active and Low-Cost Hyperspectral Imaging for the Spectral Analysis of a Low-Light Environment

Hyperspectral imaging is capable of capturing information beyond conventional RGB cameras; therefore, several applications of this have been found, such as material identification and spectral analysis. However, similar to many camera systems, most of the existing hyperspectral cameras are still passive imaging systems. Such systems require an external light source to illuminate the objects, to capture the spectral intensity. As a result, the collected images highly depend on the environment lighting and the imaging system cannot function in a dark or low-light environment. This work develops a prototype system for active hyperspectral imaging, which actively emits diverse single-wavelength light rays at a specific frequency when imaging. This concept has several advantages: first, using the controlled lighting, the magnitude of the individual bands is more standardized to extract reflectance information; second, the system is capable of focusing on the desired spectral range by adjusting the number and type of LEDs; third, an active system could be mechanically easier to manufacture, since it does not require complex band filters as used in passive systems. Three lab experiments show that such a design is feasible and could yield informative hyperspectral images in low light or dark environments: (1) spectral analysis: this system's hyperspectral images improve food ripening and stone type discernibility over RGB images; (2) interpretability: this system's hyperspectral images improve machine learning accuracy. Therefore, it can potentially benefit the academic and industry segments, such as geochemistry, earth science, subsurface energy, and mining.

Defining the scope for altering rice leaf anatomy to improve photosynthesis: a modelling approach

Leaf structure plays an important role in photosynthesis. However, the causal relationship and the quantitative importance of any single structural parameter to the overall photosynthetic performance of a leaf remains open to debate. In this paper, we report on a mechanistic model, eLeaf, which successfully captures rice leaf photosynthetic performance under varying environmental conditions of light and CO₂ . We developed a 3D reaction-diffusion model for leaf photosynthesis parameterised using a range of imaging data and biochemical measurements from plants grown under ambient and elevated CO₂ and then interrogated the model to quantify the importance of these elements. The model successfully captured leaf-level photosynthetic performance in rice. Photosynthetic metabolism underpinned the majority of the increased carbon assimilation rate observed under elevated CO₂ levels, with a range of structural elements making positive and negative contributions. Mesophyll porosity could be varied without any major outcome on photosynthetic performance, providing a theoretical underpinning for experimental data. eLeaf allows quantitative analysis of the influence of morphological and biochemical properties on leaf photosynthesis. The analysis highlights a degree of leaf structural plasticity with respect to photosynthesis of significance in the context of attempts to improve crop photosynthesis.

Leaf cell-specific and single-cell transcriptional profiling reveals a role for the palisade layer in UV light protection

Like other complex multicellular organisms, plants are composed of different cell types with specialized shapes and functions. For example, most laminar leaves consist of multiple photosynthetic cell types. These cell types include the palisade mesophyll, which typically forms one or more cell layers on the adaxial side of the leaf. Despite their importance for photosynthesis, we know little about how palisade cells differ at the molecular level from other photosynthetic cell types. To this end, we have used a combination of cell-specific profiling using fluorescence-activated cell sorting and single-cell RNA-sequencing methods to generate a transcriptional blueprint of the palisade mesophyll in Arabidopsis thaliana leaves. We find that despite their unique morphology, palisade cells are otherwise transcriptionally similar to other photosynthetic cell types. Nevertheless, we show that some genes in the phenylpropanoid biosynthesis pathway have both palisade-enriched expression and are light-regulated. Phenylpropanoid gene activity in the palisade was required for production of the ultraviolet (UV)-B protectant sinapoylmalate, which may protect the palisade and/or other leaf cells against damaging UV light. These findings improve our understanding of how different photosynthetic cell types in the leaf can function uniquely to optimize leaf performance, despite their transcriptional similarities.

The Secondary Metabolites Profile in Horse Chestnut Leaves Infested with Horse-Chestnut Leaf Miner

Natural defensive substances synthesized by plants that could replace synthetic pesticides in the protection of plants against insect invasions are constantly being sought. The study assessed changes in the qualitative and quantitative composition of secondary metabolites in horse chestnut leaves collected in different locations and differing in the sensitivity of the plant to the invasion by the horse-chestnut leaf miner. An attempt was made to identify compounds that are most responsible for the increased plant resistance to this threat. Additionally, changes in the anatomy of chestnut leaves affected by the pest were presented. It was noticed that the trees differed in the composition of secondary metabolites already in the initial growing season, which should be related to the influence of habitat conditions. The analysis of the profile of the compounds in non-infested and infested horse chestnut leaves revealed a clear response of the plant to the stress factor, i.e., the foraging of the horse-chestnut leaf miner. Catechins seem to be compounds involved in plant resistance. The leaf anatomy showed enhanced accumulation of phenolic compounds at the pest foraging sites. Hypertrophy and thickened and cracked cell walls of the spongy parenchyma were visible in the vicinity of the mines.

Enhancement of Photosynthetic Capacity in Spongy Mesophyll Cells in White Leaves of Actinidia kolomikta

Considering that Actinidia kolomikta bears abundant white leaves on reproductive branches during blossoming, we hypothesized that the white leaves may maintain photosynthetic capacity by adjustments of leaf anatomy and physiological regulation. To test this hypothesis, leaf anatomy, gas exchange, chlorophyll a fluorescence, and the transcriptome were examined in white leaves of A. kolomikta during flowering. The palisade and spongy mesophyll in the white leaves were thicker than those in green ones. Chloroplast development in palisade parenchyma of white leaves was abnormal, whereas spongy parenchyma of white leaves contained functional chloroplasts. The highest photosynthetic rate of white leaves was ~82% of that of green leaves over the course of the day. In addition, the maximum quantum yield of PSII (F _v/F _m) of the palisade mesophyll in white leaves was significantly lower than those of green ones, whereas F _v/F _m and quantum yield for electron transport were significantly higher in the spongy mesophyll of white leaves. Photosynthetic capacity regulation of white leaf also was attributed to upregulation or downregulation of some key genes involving in photosynthesis. Particularly, upregulation of sucrose phosphate synthase (SPS), glyeraldehyde-3-phosphate dehydrogenase (GAPDH) and RuBisCO activase (RCA) in white leaf suggested that they might be involved in regulation of sugar synthesis and Rubisco activase in maintaining photosynthetic capacity of white leaf. Conclusions: white leaves contained a thicker mesophyll layer and higher photosynthetic activity in spongy parenchyma cells than those of palisade parenchyma cells. This may compensate for the lowered photosynthetic capacity of the palisade mesophyll. Consequently, white leaves maintain a relatively high photosynthetic capacity in the field.

Structural organization of the spongy mesophyll

Many plant leaves have two layers of photosynthetic tissue: the palisade and spongy mesophyll. Whereas palisade mesophyll consists of tightly packed columnar cells, the structure of spongy mesophyll is not well characterized and often treated as a random assemblage of irregularly shaped cells. Using micro-computed tomography imaging, topological analysis, and a comparative physiological framework, we examined the structure of the spongy mesophyll in 40 species from 30 genera with laminar leaves and reticulate venation. A spectrum of spongy mesophyll diversity encompassed two dominant phenotypes: first, an ordered, honeycomblike tissue structure that emerged from the spatial coordination of multilobed cells, conforming to the physical principles of Euler's law; and second, a less-ordered, isotropic network of cells. Phenotypic variation was associated with transitions in cell size, cell packing density, mesophyll surface-area-to-volume ratio, vein density, and maximum photosynthetic rate. These results show that simple principles may govern the organization and scaling of the spongy mesophyll in many plants and demonstrate the presence of structural patterns associated with leaf function. This improved understanding of mesophyll anatomy provides new opportunities for spatially explicit analyses of leaf development, physiology, and biomechanics.

Desiccation of the leaf mesophyll and its implications for CO2 diffusion and light processing

Leaves balance CO₂ and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress-induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non-stressed and drought conditions using a combination of light microscopy, X-ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low-mesophyll porosity that were associated with greater gas-phase diffusion (g_IAS ), stomatal and mesophyll (g_m ) conductances and carboxylation capacity. More and highly-packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited drought-induced reductions in mesophyll cell volume, yet the associated increases in porosity and g_IAS were obscured by declines in biochemical activity that decreased g_m . Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought-induced changes in leaf structure impacted performance via imposing species-specific limitations to light absorption, gas exchange and hydraulics.

Leaf microscopy applications in photosynthesis research: identifying the gaps

Leaf imaging via microscopy has provided critical insights into research on photosynthesis at multiple junctures, from the early understanding of the role of stomata, through elucidating C4 photosynthesis via Kranz anatomy and chloroplast arrangement in single cells, to detailed explorations of diffusion pathways and light utilization gradients within leaves. In recent decades, the original two-dimensional (2D) explorations have begun to be visualized in three-dimensional (3D) space, revising our understanding of structure-function relationships between internal leaf anatomy and photosynthesis. In particular, advancing new technologies and analyses are providing fresh insight into the relationship between leaf cellular components and improving the ability to model net carbon fixation, water use efficiency, and metabolite turnover rate in leaves. While ground-breaking developments in imaging tools and techniques have expanded our knowledge of leaf 3D structure via high-resolution 3D and time-series images, there is a growing need for more in vivo imaging as well as metabolite imaging. However, these advances necessitate further improvement in microscopy sciences to overcome the unique challenges a green leaf poses. In this review, we discuss the available tools, techniques, challenges, and gaps for efficient in vivo leaf 3D imaging, as well as innovations to overcome these difficulties.

Lighting from Top and Side Enhances Photosynthesis and Plant Performance by Improving Light Usage Efficiency

Light is a critical environmental factor that influences plant growth and development, ranging from seed germination to flowering and fruiting. This study was carried out to explore how the optimal combination of various lighting directions increases the light usage efficiency and influences the plant morphophysiology, by investigating the plant growth parameters, leaf anatomy, epidermal morphology, stomatal properties, chlorophyll content, key physiological changes, and correlated gene expressions. In closed-type plant growth chambers, rooted cuttings of two chrysanthemum (Chrysanthemum morifolium Ramat.) cultivars, “Pearl Egg” and “Gaya Glory”, were subjected to a 10-h photoperiod with 600 μmol∙m⁻²·s⁻¹ photosynthetic photon flux density (PPFD) provided by light-emitting diodes (LEDs) in each light-direction combination (top (1/1) (T), top (1/2) + side (1/2) (TS), top (1/2) + bottom (1/2) (TB), side (1/2) + bottom (1/2) (SB), and top (1/3) + side (1/3) + bottom (1/3) (TSB)). The TS lighting significantly enhanced the morphophysiological performance, compared to the other lighting direction combinations. Notably, the excellent branch formation and earlier flowering were induced by the TS lighting in both “Pearl Egg” and “Gaya Glory” plants.

Leaf reflectance can surrogate foliar economics better than physiological traits across macrophyte species

BACKGROUND

Macrophytes are key players in aquatic ecosystems diversity, but knowledge on variability of their functional traits, among and within species, is still limited. Remote sensing is a high-throughput, feasible option for characterizing plant traits at different scales, provided that reliable spectroscopy models are calibrated with congruous empirical data, but existing applications are biased towards terrestrial plants. We sampled leaves from six floating and emergent macrophyte species common in temperate areas, covering different phenological stages, seasons, and environmental conditions, and measured leaf reflectance (400-2500 nm) and leaf traits (dealing with photophysiology, pigments, and structure). We explored optimal spectral band combinations and established non-parametric reflectance-based models for selected traits, eventually showing how airborne hyperspectral data could capture spatial-temporal macrophyte variability.

RESULTS

Our key finding is that structural-leaf dry matter content, leaf mass per area-and biochemical-chlorophyll-a content and chlorophylls to carotenoids ratio-traits can be surrogated by leaf reflectance with normalized error under 17% across macrophyte species. On the other hand, the performance of reflectance-based models for photophysiological traits substantively varies, depending on macrophyte species and target parameters.

CONCLUSIONS

Our main results show the link between leaf reflectance and leaf economics (structure and biochemistry) for aquatic plants, thus envisioning a crucial role for remote sensing in enhancing the level of detail of macrophyte functional diversity analysis to intra-site and intra-species scales. At the same time, we highlighted some difficulties in establishing a general link between reflectance and photosynthetic performance under high environmental heterogeneity, potentially opening further investigation directions.

Side Lighting Enhances Morphophysiology by Inducing More Branching and Flowering in Chrysanthemum Grown in Controlled Environment

Light is one of the most important factors that influence plant growth and development. This study was conducted to examine how lighting direction affects plant morphophysiology by investigating plant growth parameters, leaf anatomy, epidermal cell elongation, stomatal properties, chloroplast arrangement, and physiological changes. In closed-type plant factory units, the rooted cuttings of two chrysanthemum (Chrysanthemum morifolium Ramat.) cultivars, ‘Gaya Glory’ and ‘Pearl Egg’, were subjected to a 10 h photoperiod with a 300 μmol∙m⁻²∙s⁻¹ photosynthetic photon flux density (PPFD) provided by light-emitting diodes (LEDs) from three directions relative to the plant including the top, side, and bottom. Compared to the top or bottom lighting, the side lighting greatly enhanced the plant growth, improved the leaf internal structure and chloroplast arrangement, induced small stomata with a higher density, and promoted stomatal opening, which is associated with an increased stomatal conductance and photosynthetic efficiency. It is worth noting that the side lighting significantly enhanced the induction of branching and flowering for both cultivars., The plants grown with side lighting consistently exhibited the greatest physiological performance. We conclude that the lighting direction had a profound effect on the morphophysiological characteristics of chrysanthemum, and that side lighting dramatically promoted their growth and development, especially in their branching and flowering.

Genome streamlining in a minute herbivore that manipulates its host plant

The tomato russet mite, Aculops lycopersici, is among the smallest animals on earth. It is a worldwide pest on tomato and can potently suppress the host's natural resistance. We sequenced its genome, the first of an eriophyoid, and explored whether there are genomic features associated with the mite's minute size and lifestyle. At only 32.5 Mb, the genome is the smallest yet reported for any arthropod and, reminiscent of microbial eukaryotes, exceptionally streamlined. It has few transposable elements, tiny intergenic regions, and is remarkably intron-poor, as more than 80% of coding genes are intronless. Furthermore, in accordance with ecological specialization theory, this defense-suppressing herbivore has extremely reduced environmental response gene families such as those involved in chemoreception and detoxification. Other losses associate with this species' highly derived body plan. Our findings accelerate the understanding of evolutionary forces underpinning metazoan life at the limits of small physical and genome size.